1. Field of the Invention
Embodiments described herein relate generally to a magnetic resonance imaging method, a magnetic resonance imaging apparatus, and a control device of a magnetic resonance imaging apparatus.
2. Description of the Related Art
MRI is an imaging method which magnetically excites nuclear spin of an object (a patient) set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation. The aforementioned MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance signal.
A known cause of image quality degradation of MRI is distortion of a gradient magnetic field distribution. Ideally, the gradient magnetic field is distributed in the slice selection direction, the phase encoding direction and the frequency encoding direction in such a manner that the magnetic field intensity linearly varies with the position in the direction of application, for example. In actuality, however, a pulse current supplied to a gradient magnetic field coil causes an eddy current. The eddy current induces a magnetic field, and the magnetic field is added to the gradient magnetic field to cause distortion of the gradient magnetic field distribution.
To correct the input gradient magnetic field waveform, a typical eddy-current compensation is designed to compensate for only the primary component of the magnetic field induced by the eddy current, which spatially linearly varies. Therefore, the eddy-current compensation cannot compensate for the secondary and higher-order components of the magnetic field induced by the eddy current. To solve the problem, there is a technique of compensating for the secondary and higher-order components of the magnetic field induced by the eddy current by applying a current to higher-order shim coils.
In addition, Japanese Patent Laid-Open No. 3-195539 discloses an arrangement that compensates for a magnetic field induced by an eddy current by selecting one or more of a plurality of current compensation circuits previously provided.
In general, the distortion of the gradient magnetic field distribution becomes more significant as the distance from the center of the magnetic field increases. Accordingly, the conventional technique that compensates for only the primary component of the magnetic field induced by the eddy current cannot prevent image quality degradation due to the distortion of the magnetic field distribution caused by the secondary and higher-order components of the magnetic field induced by the eddy current, in particular when the imaging region is located off-center. The term off-center means at a position away from the center of the magnetic field.
The conventional technique that uses higher-order shim coils to compensate for the secondary and higher-order components of the magnetic field induced by the eddy current has a disadvantage that a waiting time of several seconds is needed to avoid coupling with other coils or other effects, for example. Furthermore, in some cases, channels for the higher-order shim components may not correspond to channels for the higher-order components of the magnetic field induced by the eddy current. For example, even if the shim coils include coils for compensating for the secondary components, such as XZ and YY, the magnetic field induced by the eddy current cannot be sufficiently compensated for if the magnetic field induced by the eddy current has a significant third-order or fourth-order component.